Based on these new findings and those reported previously[16,34,36,37], we propose that IL-11-expressing breast cancer cells cause increased osteoclast formation and bone resorption by two distinct mechanisms: a) the tumor cells produce IL-11 which in turn stimulate the production of RANKL by stromal cells/osteoblasts in the bone microenvironment, and b) tumor cell-derived IL-11 also augments the pool of osteoclast progenitor cells to increase the extent of osteoclastogenesis. The gene discussed is TNFSF11; the disease is neoplasm.